The effects of a radio channel upon a radio signal transmitted on the radio channel are well known. Well-known effects include poor signal quality due to low signal to noise ratio (SNR), adjacent and co-channel interference and multi-path propagation. Where extreme distance is a factor a poor SNR may be due to thermal noise. Where distance is slight a poor SNR may be due to competing signals on the same or an adjacent channel.
Multi-path propagation, on the other hand, produces an effect on the signal characterized by multiple copies of the signal being presented to a receiver at slightly different times and with slightly different phases. In extreme cases multiple copies of a signal may arrive at a receiver offset over a time interval comparable to a symbol transmission rate.
The problem of multi-path propagation results in a summation of signals being presented to a receiver that may bear little resemblance to the originally transmitted signal. Where either the transmitter or receiver is moving (e.g., a radiotelephone in an automobile) the problem of multi-path propagation may be further aggravated in that the effects on the signal may also vary with physical location.
Past efforts to improve decoding of signals subject to low SNR and the effects of multi-path propagation have included adding a training (synchronization) sequence to the beginning of data transmission within a frame of information and cross-correlating the received signal against the known training sequence. The results of the cross-correlation are then used to characterize and compensate for the affects of the transmission channel.
While characterizing the transmission channel is effective for short periods, such characterization may not be effective for frames having durations of several milliseconds. For frames of longer duration the transmitter and receiver may change physical locations thereby changing the transmission channel and altering transmission characteristics.
Past efforts to improve performance under such conditions have included the systems discussed in IEEE Transactions On Information Theory, January 1973, pgs. 120-124, F. R. Magee Jr. and J. G. Proakis: "Adaptive Maximum Likelihood Sequence Estimation for Digital Signaling in the Presence of Intersymbol Interference". The Magee and Proakis article teaches of a system having an adaptive filter used in conjunction with a viterbi decoder. The values of the adaptive filter are determined upon detection of a training sequence and subsequently modified based upon each new symbol output from the viterbi decoder.
While the Magee and Proakis system has been effective, the effectiveness of the adaptive filter is dependent upon detection and timing of the training sequence. Where the training sequence is corrupted or subject to superposition of multiple copies of the training sequence then the effectiveness of the adaptive filter declines because of synchronization deficiencies and dispersion of signal energy. Because of the importance of maximum likelihood sequence estimators a need exists for a better method of synchronization with, and optimization of signal energy within the training sequence.